@r00s7a This is most likely an issue with your network infrastructure and not fog. The DHCP offers come from your primary dhcp server. If that DHCP server is not FOG Server then its not something you can solve on the fog server.
Where we have seen sometimes they pxe boot and sometimes not is if you have two dhcp servers on your network. Possibly in a primary / secondary configuration. Confirm that all dhcp servers have dhcp options 66 and 67 set because these settings are not replicated between windows dhcp servers.